The long-term objective of this project is a thorough understanding of the neural mechanisms of binaural interaction in the auditory brainstem nuclei, particularly as they relate to sound localization and other aspects of binaural hearing. The basic approach in this project entails electrophysiological recordings from single neurons using extracellular techniques while employing a broad array of complex stimuli synthesized under computer control. The stimuli are carefully chosen to help elucidate neuronal function and/or to mimic psychophysical phenomena for it is important to interpret and relate physiological results to psychophysics. The present application proposes to continue on-going acute studies as well as to begin behavioral studies of sound localization. There are three general aims. One (1) is to study the physiological correlates of the precedence effect by recordings in the inferior colliculus, as well as in the auditory nerve and cochlear nucleus, using both earphones and free field stimulation. The underlying hypothesis is that the discharge of some cells in the inferior colliculus is related to the perceived location of the sound source. Two (2) is to study the encoding of interaural spectral differences in the inferior colliculus using the head-related transfer functions recorded in project IV of this Program. These studies will allow direct correlation of the responses of cells to free field stimuli, as simulated by the head-related transfer functions, and to traditional measures of binaural interaction measured under headphones. Three (3) is the development of a new behavioral preparation to train cats to look at sound sources and to use this preparation for a number of important studies. The movements of the cats pinnae during active localization will be studied, confirmation of the perception of the virtual space stimuli will be sought by delivering the signals via headphones to cats that are localizing sounds, the degree to which the precedence effect holds for cats will be tested, and recordings will be made from single cells in the inferior colliculus that are thought to be important for encoding the direction of a sound during active localization. Spatial hearing is an important basic function of the auditory system: defects in binaural function in human patients can lead to considerable difficulty in detecting signals embedded in noise, such as understanding a conversation in a noisy room, which is perhaps the mst common complaint of the hearing-impaired and can lead to severe social withdrawal.